Over the 120 years that motor vehicles have existed, their suspension systems have evolved considerably. This evolution has been driven primarily by a continuously developing understanding of the influence exerted by the suspension on the behaviour of the vehicle, and consequently the properties required of the suspension in order to ensure a vehicle with favourable driving characteristics.
In modern vehicles, a suspension system can be described as a system of components connecting a wheel carrier incl. a wheel to the vehicle structure.
The suspension system of a modern vehicle is typically a linkage system consisting of rigid load-bearing links capable of articulating about joints which usually takes the form of rubber bushings.
Suspension architectures can be categorised into architectures which vary in their complexity and as a consequence thereof, their cost and performance.
Examples of common suspension architectures are the MacPherson strut, the double wishbone, the live axle and the twist beam axle.
Suspension architectures can be further divided into three main categories according to their degree of interconnection between the left and right hand wheels of an axle, the categories are:                independent,        semi-dependent, and        dependent.        
In an independent suspension system, the left and right hand wheels have no connection between them (apart from contingent anti-roll bars which are typically ignored under these definitions) and have therefore no influence on one another's movement. The independent suspension system is typically applied forward and/or aft of premium class vehicles and/or high performance vehicles.
In a dependent suspension system, the left and right hand wheel carriers are rigidly connected to one another. The dependent suspension system is typically applied forward and/or aft of heavy commercial vehicles.
In a semi-dependent suspension system, the left and right hand wheel carriers are not rigidly connected. The connection between the left and right hand wheels is such that sufficient stiffness is obtained to allow the movement of one wheel to significantly affect that of the opposite wheel. Such suspension system may encompass a so-called twist beam axle, and is the most common architecture in this category. The semi-dependent suspension system, and in particular the twist beam axle is, typically due to cost considerations, applied aft of small and medium class vehicles, while the forward system typically comprise an independent suspension system, e.g. such as the abovementioned MacPherson strut.
As per the above, the front suspension systems of modern passenger cars are, almost without exception, independent suspensions systems (again, apart from contingent anti-roll bars), whilst the rear suspension systems tend to be divided between independent systems on higher cost cars and semi-dependent systems on lower cost cars.
The modern suspension system has various functions; some are obvious, others are less so. Its primary functions are:                structural: Transmission of forces generated at the contact patch between the tyre and the road to the main mass of the vehicle,        handling: Ensuring that, when the wheel is subject to the forces generated by events such as braking, cornering and acceleration, or when it is subject to vertical displacements, a precise and predefined displacement of the wheel takes place relative to the vehicle structure. For example, during braking and cornering it is necessary for the wheels to steer very slightly in proportion to the force generated by the tyre to ensure a vehicle behaviour which results in both comfort of the passengers and also safe and stable manoeuvring, and        comfort: Isolating (preferably) the passenger compartment of the vehicle from the irregularities of the road surface over which the vehicle is moving.        
The characteristics of the comfort-increasing “vibration filter” are different according to the frequency (e.g. resulting from the speed of a vehicle), amplitude and direction of the inputs. The principal requirements being that a suspension system has to cope with following strains or loads originating from either irregularities in the surface over which a vehicle is travelling, or from strains or loads originating from alterations in speed and course:                A. Low frequency (≅1 Hz), high amplitude (±100 mm) in a vertical direction,        B. Medium frequency (≅10 Hz), moderate amplitude (5-10 mm), in the vertical and longitudinal directions, and        C. High frequency (>50 Hz), low amplitude (<2 mm) in all directions.        
Requirement A is by non-experts considered to be the sole function of the suspension system, and indeed the low frequency vertical isolation was the primary motive for the development of the original suspension systems on horse-drawn carriages and the like.
With today's greater driving speeds and high comfort expectations, the suspension systems are required also to cope with the requirements under B and C above, and it is in fact under these requirements, the true challenge in the design of modern passenger car suspensions arises.
As mentioned above, in the early days low frequency vertical isolation was the sole requirement of the suspension; a live axle with a simple linkage, and some form of springing, and later, damping being all that was required.
It was later realised that changes in wheel angles during the wheels vertical motion was critical to the handling performance of the vehicle.
In particular, some of the angles of interest are:                camber angle, which is the angle between a substantially vertical axis of the wheel and the vertical axis of the vehicle when viewed from the front or rear. The camber angle is in other words the wheels vertical inclination, in or out, transverse to the vehicles longitudinal axis. The angle may be obtained from static geometry of the suspension components and/or from kinematic and compliant effects of the suspension components, and        toe angle, which is the angular position of the wheel with respect the longitudinal axis of the vehicle. The angle may be positive, i.e. the front of the wheels are closer together than the back of the wheels (toe-in) or negative, i.e. the back of the wheels are closer together than the front of the wheels (toe-out). The angle may be obtained from static geometry of the suspension components and/or from possible kinematic and compliant effects of the suspension components.        
The increased understanding of the desired kinematics of the wheel necessitated use of more complex linkages to control the wheel, and ultimately the use of separate linkages to control each wheel, such as independent suspension systems was conceived.
Later still came the understanding of how the wheel displaced under loads in a horizontal plane, e.g. fore/aft loads during braking or acceleration and lateral loads during cornering etc. Designers realized that changes in wheel angles as a result of wheel displacements originating from one or more of these loads was also critical to the vehicles handling response, and consequently to driver enjoyment and vehicle stability and safety.
This led to focus being placed on the compliant properties of the suspension linkage, and as a consequence, on the stiffness of the rubber bushings providing articulation between the links. Along with this came the realisation that certain types of linkage were preferred over others in order to meet the compliant requirements now being placed on the suspension.
More recently, the importance of the fore/aft flexibility, i.e. the longitudinal compliance, of the suspension system has been appreciated. It is now known that this property is vital to the suspension's ability to absorb impacts, such as bumps and potholes, and provide good road noise isolation. This is now a fundamental requirement of the suspension system, and whilst this “discovery” did not specifically lead to the development of new suspension linkages, it has in recent years heavily influenced the car manufacturer's choice of suspension architecture.
US 2004/0160033 A (BRIDGESTONE CORP.) teaches a torsion beam type suspension device comprising a pair of trailing arms spaced from each other axially of an axle and connected at front ends thereof to a car body and at rear ends thereof supporting wheels through brackets demonstrating compliant behaviour with respect to the trailing arms. A torsion beam extending axially of the axle and interconnecting the pair of trailing arms at connecting portions. The rear ends of the pair of trailing arms are connected to the brackets through rubber cushions, whereby the steering stability, according to the document, is improved.
US 2007/0052192 A (KAWANOBE) teaches an axle which rotatable supports a wheel being elastically coupled to trailing arms of a motor vehicle via an upper connecting point and first and second lower connecting points, where the trailing arms are elastically coupled to a body of the vehicle. The first and second lower connecting points are positioned lower than the upper connecting point and are mutually arranged to open a gap in a front-to-rear direction of the vehicle. The first and second lower connecting points have elastic members resulting in a stringency of the wheel carrier in all directions, and arranged so that principal elastic axes of their respective elastic members intersect at a location that is outside, in the transverse direction of the vehicle, of a grounding point of the wheel. According to the document, providing a suspension system with principal elastic axes intersecting at a location outside, in the transverse direction of the vehicle, of a grounding point of the wheel increases the running stability of the vehicle without relying on the rigidity of the elastic members, resulting in increased freedom in design of the elastic members.
US 2006/082091 A (MOSLER) teaches an independent wheel suspension comprising a wheel guiding strut which is mounted on a vehicle body. A wheel carrier is supported on the strut by several elastic pivoting bearings. According to the document, a high degree of driving comfort is provided by a controlled accommodation of lateral, longitudinal and vertical forces.
EP 1640249 A1 (PSA) teaches a vehicle having a twist beam axle base suspension with a stub axle which by means of a compliant system incorporating rubber blocks is mounted to the twist beam axle. The deflection of the stub axle is controlled by the rubber blocks and limited in its extreme positions by pins arranged in sliding slots. The document does not suggest creating an additional steer axis, as the elastic mechanism appears to be an axis trans-verse to the axis of the vehicle.
GB 2270508 A (McLaren) teaches an independent front suspension for a vehicle wherein each wheel suspension, which constitutes wishbones etc., is mounted in its entirety on a vertical sub frame which again is mounted to the vehicle chassis by means of four rubber bushings, all of which having higher radial stiffness than axial stiffness. The wishbones are pivotally and noncompliant mounted on the sub-frame. The compliance of the suspension system derives from the four rubber bushings connecting the sub-frame to the structure of the vehicle. The rubber bushings each comprise a generally hollow cylindrical elastomeric bush which is stiff radially thereof, but soft axially thereof. The axes of maximum stiffness of the bushes intercept one another to define a shear centre of the arrangement which lies on the ground. The bushings for the sub-frame are arranged such that their axes all lie in a longitudinal/vertical plane, the direction of each axis being tangential to the contact patch of the tyre. In this way, an axis of low torsional stiffness is created between the suspension linkage (and therefore the wheel carrier, as the bushings of the suspension linkage itself are all designed to be stiff) and the vehicle chassis. The suspension system seeks to generate a transverse axis of low torsional stiffness below the wheel centre in order to improve comfort. However, the document fails to suggest a vertical axis about which the wheel is allowed to move in order to improve to improve toe-in/out characteristics.